Exhaust system for an internal combustion engine

ABSTRACT

An exhaust gas aftertreatment system for an internal combustion engine comprises a conduit for carrying a flowing exhaust gas, at least one filter for particulate matter, an oxidation catalyst for oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO 2 ) which catalyst is disposed upstream of the at least one filter, means for limiting flow of an exhaust gas in the conduit comprising a cut-off valve disposed in the conduit and control means selectively to operate the means for increasing the back-pressure thereby to increase the temperature in the system.

The present invention relates to measures for preventing or reducingnitrogen dioxide (NO₂) slip in an exhaust gas aftertreatment system foran internal combustion engine comprising a catalyst for oxidisingnitrogen monoxide (NO) in the exhaust gas to NO₂ and at least one filterfor particulate matter disposed downstream of the catalyst. Inparticular the invention relates to a system for actively regeneratingthe at least one filter.

EP-A-0341832 describes a process of combusting diesel soot trapped on afilter in NO₂ at temperatures of up to 400° C. by oxidising NO in dieselexhaust gas to NO₂ over an oxidation catalyst disposed upstream of thefilter. A device embodying this process is marketed by Johnson Mattheyas the CRT®. The entire contents of EP-A-0341832 are incorporated hereinby reference.

A problem with the process in use is that the rates of NO oxidation andsoot combustion in NO₂ are relatively low at lower exhaust gastemperatures encountered in certain situations. These situations caninclude periods of idling and slow driving in traffic. It is thereforedesirable to raise the temperature of the CRT® device during periods oflower exhaust gas temperatures to increase the combustion of trappedsoot, levels of which would otherwise gradually increase. Raising thetemperature of the CRT® device to aid in the combustion of trapped sootis known as “active regeneration”.

A number of modes of increasing the temperature of components of anexhaust gas aftertreatment system are known. Non-limiting examples ofthese include employing an electrically heated catalyst, introducingunburnt hydrocarbon into an exhaust gas to create an exotherm as thehydrocarbon is combusted over a catalyst in the system and secondaryinjection of air into the system to aid combustion of unburnthydrocarbon. However, there are problems associated with these knownmethods, including increased fuel penalty and the requirement forcomplicated and expensive hardware and control means.

We have investigated ways of actively regenerating a CRT® device andhave now found that the efficiency of the CRT® process can be improvedby selectively increasing the back-pressure in the exhaust system. Webelieve that this is for at least two reasons. Firstly, increasing theback-pressure in the system can result in an increase in the exhaust gastemperature as the engine is made to work harder. This increase in theexhaust gas temperature can promote the combustion of soot on the filterin NO₂.

Secondly, the increase in exhaust gas temperature can thermodynamicallypromote the oxidation of NO to NO₂. This in turn can increase the rateof combustion of soot on the filter in NO₂.

We believe that the method of the invention can also be used to activelyregenerate all forms of catalysed and non-catalysed particulate filtersin exhaust gas aftertreatment systems, and for all internal combustionengines employing the CRT® process.

According to one aspect, the invention provides an exhaust gasaftertreatment system for an internal combustion engine, comprising aconduit for carrying a flowing exhaust gas, at least one filter forparticulate matter, an oxidation catalyst for oxidising NO to NO₂, whichcatalyst is disposed upstream of the at least one filter, means forlimiting flow of an exhaust gas in the conduit thereby to increaseback-pressure in the system, which flow limiting means comprising acut-off valve disposed in the conduit, a sensor for detecting an amountof NO₂ in exhaust gas downstream of the filter, and control meansarranged selectively to operate the flow limiting means when the amountof NO₂ detected in the exhaust gas is at or above a pre-determinedvalue, thereby to increase the temperature in the system andconsequently to increase the rate of reaction between NO₂ andparticulate matter.

In one embodiment the flow limiting means can substantially prevent flowof exhaust gas in the system.

The cut off valve can be positioned in any suitable position dependinge.g. on space; and/or prevention of heat loss in the system forcombusting soot and/or oxidising NO. In certain embodiments, the valvecan be disposed upstream of the NO oxidation catalyst; downstream of thefilter; or between the filter and the NO oxidation catalyst.

At its simplest, the invention provides a switch for operating the meansfor increasing the back-pressure, e.g. in response to a warning light ona vehicle dashboard, and the switch is operated by the driver. Operatingthe flow limiting means increases back-pressure in the system.Increasing back-pressure in the system generally results in an increasein the temperature of the filter. In turn this can raise the rate ofcombustion in oxygen or NO₂ of particulate matter trapped on the atleast one filter. Increasing the exhaust gas temperature canthermodynamically increase the rate of NO oxidation over the catalyst.

In an illustrative embodiment, the control means can be arrangedselectively to operate the flow limiting means during engine idling.

In a further embodiment, the control means is arranged selectively tooperate the flow limiting means when the detected temperature of the oreach filter and/or exhaust gas e.g. using a thermocouple or infra-redsensor, is at up to 400° C.

In a further embodiment, the system includes a sensor for sensingback-pressure in the system as an indication of particulate matterloading on the at least one filter wherein the control means alsooperates the flow limiting means when the detected particulate matterloading on the or each filter exceeds a pre-determined value. This canbe detected, for example, using the back-pressure in the system.Alternatively, the flow limiting means can be operated in response to adetected condition in an engine map of accelerator position or elapsedtime, in addition to when NO₂ slip is detected. In an illustrativeembodiment, however, the means for increasing the back-pressure isdeployed only when the engine is at idle, since deployment duringdriving can cause driveability problems and can also give very highengine out smoke levels.

In a further aspect, the means for limiting exhaust gas flow is anengine brake. It is common practice as a safety feature and to improvefuel efficiency for a vehicle to include an engine brake whereby liftingoff from the accelerator pedal during driving leads to fuel cut-off.Such known engine brakes can include U.S. Pat. No. 4,149,618, the entirecontents of which are incorporated herein by reference. Where thepresent invention utilises an engine brake which is ordinarily used on acertain vehicle, it may be unnecessary to provide new, potentiallycomplicated and expensive hardware to adopt the invention. Instead itmay be possible to integrate the invention into an existing vehicle bysimple reprogramming of the engine brake control means, e.g. enginemanagement unit including an electronic control unit (ECU) or computerchip.

The at least one filter need not be catalysed, but in embodimentsaccording to the invention it can include any catalyst capable ofcatalysing combustion of particulate matter in oxygen or NO₂. Forexample, in one illustrative embodiment, the or each filter may compriseat least one platinum group metal, such as platinum, palladium, rhodium,ruthenium or iridium. Alternatively, a mixed caesium/lanthanum/vanadiumpentoxide catalyst can be used.

Application of the present invention to the CRT® is particularlyadvantageous for at least three reasons. Firstly, an increase inback-pressure in the system causes an increase in the temperature of thesystem as a whole, thereby increasing the rate of NO oxidation over thecatalyst. Thus, more NO₂ is available to combust the trapped particulatematter. Secondly, increased filter temperature leads to an increase inthe rate of reaction between NO₂ and trapped particulate matter.Thirdly, it causes engine-out NOx levels to increase thereby alsoincreasing the NOx available for oxidation to combust trappedparticulate matter.

The exhaust system of the present invention can be applied to anyinternal combustion engines. For example, the engine can be a lean burnengine such as a lean burn gasoline engine. e.g. a gasoline directinjection (GDI) engine, or a diesel engine. Where the engine is a dieselengine, in an illustrative embodiment it is a heavy-duty diesel engineaccording to the relevant EU, US Federal or Californian legislation. Forexample, the present invention has particular utility in heavy-dutydiesel vehicles operating in built up areas and city centres andinvolving frequent idling and stop-start driving. Examples of such usesinclude mass transit vehicles such as buses and refuse tucks.

According to a further aspect, the invention provides a method ofcontrolling NO₂ slip above a pre-determined value downstream of at leastone filter for particulate matter in an exhaust gas aftertreatmentsystem of an internal combustion engine, comprising the steps ofcollecting particulate matter from the exhaust gas on at least onefilter, catalytically oxidising NO to NO₂, combusting particulate matteron the filter in the NO₂, detecting the amount of NO₂ downstream of thefilter and selectively limiting the flow of the exhaust gas in thesystem with a flow limiting means comprising a cut-off valve thereby toincrease the temperature in the system and consequently to increase therate of reaction between NO₂ and particulate matter when the amount ofNO₂ detected is at or above a pre-determined value.

The method of the present invention is for controlling NO₂ slip in anexhaust system. NO₂ is an irritant to mucous membranes, e.g. eyes, noseand respiratory passages, and its release into the atmosphere isundesirable. The present invention is used to reduce the level of NO₂released into the atmosphere by increasing the back-pressure in thesystem when values of NO₂ detected downstream of a filter disposed inthe exhaust system are equal to or exceed a pre-determined value.Increasing the back-pressure in the system results in an increase in thetemperature of the filter, which in turn improves the rate of reactionbetween NO₂ and particulate matter over the filter. Thus NO₂ slip isreduced by increasing the rate of reactions that remove it.

In order that the invention may be more fully understood, the followingExample is provided by way of illustration only and by reference to theaccompanying drawings, in which:

FIG. 1 is a trace of temperature (° C.) and engine speed (rpm) againsttime (seconds) showing the effect of increasing the back-pressure in anexhaust system on temperatures within a CRT® system;

FIG. 2 is a trace of NO₂ (ppm), temperature (° C.) and engine speed(rpm) against time (seconds) showing the effect of increasing theback-pressure in an exhaust system on the efficiency of NO₂ generationand NO₂ usage within the CRT® system; and

FIG. 3 is a trace of NO₂ used (ppm) and engine speed (rpm) against time(seconds) showing the effect of increasing the back-pressure in anexhaust system on the amount of NO₂ used within the CRT® system.

EXAMPLE

The effect of increasing the back-pressure in an exhaust system toregenerate a particulate matter filter according to the invention hasbeen demonstrated on an engine bench using a 12-litre turbocharged,intercooled engine. The exhaust system included a CRT® unit as describedin EP-A-0341832. The engine was run to simulate e.g. a city centre busdriving cycle, involving driving between bus stops, punctuated byperiods of engine idle at the bus stops. It is preferable to deploy theengine brake only when the bus is at idle, since deployment while thebus is moving could lead to driveability issues. The cycle involvedhigh-speed (1600 rpm) engine conditions, corresponding to drivingbetween bus stops, and low speed (600 rpm) corresponding to engineidling at the bus stops. As stated above, the engine brake was onlydeployed at the 600 rpm engine condition. FIG. 1 shows the effect ofdeploying the engine brake under idle conditions (e.g. when the bus hasstopped to pick up passengers) on the temperatures within the system. Itcan be seen that the temperature at the inlet to the catalyst of theCRT® system increases as a result of the application of the enginebrake. The peak temperature during the cycle increases from 310° C. to330° C. when the brake is deployed. Similarly, the temperaturedownstream of the filter increases; the peak temperature increases from295° C. to 315° C. upon engine brake application. This 20° C. increasein temperature can significantly enhance the operation of the CRT®system.

As FIG. 2 shows, the amount of NO₂ generated by the catalyst increaseswhen the engine brake is applied. This is particularly apparent at theidle condition, where the amount of NO₂ generated increases from 250 ppmto 350 ppm when the engine brake is deployed. (Note that the engine-outNOx level increased from 350 ppm to 400 ppm when the engine brake wasdeployed; this NOx is predominantly in the form of NO. Therefore, theefficiency of the conversion of engine-out NO into NO₂ under the idlecondition increased from 71% to 88% when the engine brake was deployed,due to the increase in catalyst temperature referred to above). FIG. 2also shows that the extra NO₂ generated under this condition reacts withcarbon in the filter, since there is no increase in the NO₂ downstreamof the filter. Indeed, the amount of NO₂ downstream of the filter isactually decreased when the engine brake is applied, demonstrating thatthe increase in temperature associated with the application of theengine brake is leading to a significant increase in the rate ofreaction between NO₂ and the carbon in the filter. That is, there is ademonstrable increase in the amount of NO₂ consumed within the filterwhen the engine brake is applied, since there is an increase in theamount of NO₂ entering the filter, but a decrease in the amount of NO₂leaving the filter. Therefore, this engine brake strategy can also beused to minimise NO₂ slip.

This is shown more clearly in FIG. 3, which shows the effect of enginebrake deployment on the amount of NO₂ used within the filter (to reactwith carbon). The amount of NO₂ used in the filter is defined asfollows:NO₂ Used=NO₂ Entering the Filter−NO₂ Leaving the Filter

Therefore, it can be seen that the deployment of the engine brake leadsto an increase in the temperature of the catalyst and filter within theCRT® system. This leads to an increase in the amount of NO₂ generatedover the catalyst, and to an increase in the amount of NO₂ consumed byreaction with carbon within the filter. The deployment of the enginebrake is therefore seen to be an effective active regeneration strategyfor filter-based systems such as the CRT®.

1. An exhaust gas aftertreatment system for an internal combustionengine, comprising a conduit for carrying a flowing exhaust gas, atleast one filter for particulate matter, an oxidation catalyst foroxidising NO to NO₂, which catalyst is disposed upstream of the at leastone filter, means for limiting flow of exhaust gas in the conduitthereby to increase back-pressure in the system, which flow limitingmeans comprising a cut-off valve disposed in the conduit, a sensor fordetecting an amount of NO₂ in exhaust gas downstream of the filter, andcontrol means arranged selectively to operate the flow limiting meanswhen the amount of NO₂ detected in the exhaust gas is at or above apre-determined value, thereby to increase the temperature in the systemand consequently to increase the rate of reaction between NO₂ andparticulate matter.
 2. An exhaust system according to claim 1, whereinthe flow limiting means, when operated, substantially prevents exhaustgas flow in the conduit.
 3. An exhaust system according to claim 1,wherein the cut-off valve is disposed downstream of the filter.
 4. Anexhaust system according to claim 1, wherein the cut-off valve isdisposed upstream of the NO oxidation catalyst.
 5. An exhaust systemaccording to claim 1, wherein the cut-off valve is disposed between theno oxidation catalyst and the filter.
 6. An exhaust system accordingclaim 1, wherein the control means operates the flow limiting meansduring engine idling.
 7. An exhaust system according to claim 1, whereinthe control means operates the flow limiting means when one of thetemperature of the filter and the temperature of the exhaust gas is atup to 400° C.
 8. An exhaust system according to claim 1, furthercomprising a sensor for sensing back-pressure in the system as anindication of particulate matter loading on the at least one filter,wherein the control means also operates the flow limiting means when thedetected back-pressure in the system is at or above a pre-determinedvalue.
 9. An exhaust system according claim 1, wherein the cut-off valvecomprises an engine brake.
 10. An exhaust system according to claim 1,wherein the or each filter is catalysed.
 11. An exhaust system accordingto claim 10, wherein the filter catalyst comprises at least one platinumgroup metal.
 12. An apparatus comprising an internal combustion engineand an exhaust system according to claim
 1. 13. Apparatus according toclaim 12, wherein the internal combustion engine is a lean burn engine.14. Apparatus according to claim 13, wherein the lean burn internalcombustion engine is a heavy duty diesel engine.
 15. A method ofcontrolling NO₂ slip above a pre-determined value downstream of at leastone filter for particulate matter in an exhaust gas aftertreatmentsystem of an internal combustion engine, comprising the steps ofcollecting particulate matter from the exhaust gas on at least onefilter, catalytically oxidising NO to NO₂, combusting particulate matteron the filter with NO₂, detecting the amount of NO₂ downstream of thefilter and selectively limiting the flow of the exhaust gas in thesystem with a flow limiting means comprising a cut-off valve thereby toincrease the temperature in the system and consequently to increase therate of reaction between NO₂ and particulate matter when the amount ofNO₂ detected is at or above a pre-determined value.